Previous work using the electron microscope and negative stain had demonstrated the presence of ring structures on the surface of red cell ghosts whose membranes had been damaged by cytotoxic lymphocytes in an ADCC reaction. These ring structures with a l50A inner diameter were proposed to be membrane pores inserted into the target cell membranes during the lytic process. It was found that such ring structures are generated from material in granules of the cytotoxic lymphocytes after contact with plastic surfaces coated by antigen-antibody complexes. We therefore have begun an effort to study these granules biochemically. Tumors of rat "large granular lyumphosytes" (LGL) (which mediate both NK and ACDD killing) have been used as a source for biochemical isolation of the granules by Percoll gradients. In order to test whether or not granule components are inserted into membrane pores in target cells a means of measuring marker permeability into red cell ghosts is being developed so that we can perform experiments similar to those previously done with complement pores.